Epidermal growth factor (EGF) is a single-chain peptide (mol wt 6,045) that was originally isolated from the submandibular glands of male mice. EGF enhances the proliferation of various mammalian cells in culture but its physiological role remains to be clarified. In mice the submandibular gland is the major site of EGF production and the major source of circulating EGF. Previous studies employing experimentally induced (sialoadenectomized) EGF-deficient mice have suggested that EGF plays an important role in many physiological processes including spermatogenesis, completion of normal pregnancy and wound healing. Thus the possibility exists that EGF deficiency contributes to the pathology of disease states. Diabetes mellitus is of interest in this respect, since EGF deficiency occurs in diabetes mellitus and some of the pathology accompanying the disease includes disorders ordinarily associated with EGF deficiency. Oligozoospermia is one of the pathological complications associated with diabetes mellitus, but its etiology is not fully understood. We have shown that EGF is important for maintaining normal sperm production in mice. Removal of the submandibular gland results in the reduced level circulating EGF and about a 50% loss of mature sperm in the epididymis, whereas supplementation with EGF can fully restore the sperm counts to normal levels. Our studies have shown that in two kinds of diabetic mice, namely genetically diabetic mice and mice with streptozotocin (STZ) induced diabetes the levels of prepro-EGF-mRNA and EGF in both the submandibular gland and plasma are greatly reduced. No apparent difference in the level of kidney prepro-EGF mRNA was found between the diabetic and control mice. The epididymal sperm counts of diabetic mice were significantly lower than those of normal mice. Administration of EGF to STZ-induced diabetic mice substantially increased their sperm counts, whereas insulin treatment of diabetic mice restored the submandibular and plasma EGF concentrations and sperm counts to normal levels. The restorative effects of insulin on sperm production appeared to be mediated at least in part by EGF because of its effect was blocked by the concomitant administration of EGF antiserum. These results suggest that EGF deficiency is a possible cause for the pathogenesis of oligozoospermia in diabetic mice.